Size and activity of soil microbial communities in long-term experimental grassland plots treated with manure and inorganic fertilizers

We determined the size, activity, and affinity of the microbial community for glucose in soils from long-term experimental grassland plots. The plots had been treated annually with either farmyard manure, inorganic NPK fertilizers, farmyard manure+inorganic NPK fertilizers, (NH₄)₂SO₄ only, or no experimental amendment since 1897. The largest biomass and activity differences were between the (NH₄)₂SO₄-treated soil, which was very acid, and the rest, which were nearer neutral. In the (NH₄)₂SO₄- treated soil, the biomass C to organic C ratio was small, but overall the community had high respiratory activity per unit of biomass (qCO₂) and high overall affinity for glucose (low K(m)). The effects of the manure treatment were a greater biomass C and a lower overall glucose affinity than in the control plot. In the presence farmyard manure, NPK led to smaller biomass and a lower biomass to organic C ratio while having no significant effect on either glucose K(m) or qCO₂. In the absence of farmyard manure, NPK led to significantly greater glucose affinity but had no significant effect on the biomass, the biomass C to organic C ratio or qCO₂.